Conventionally, in cases where screws are unnecessarily removed by a third party from devices that are completed by the connection or assembly of various types of parts by means of screw attachment, disassembly of the device or the separation of parts is performed, and adjustment locations or dangerous locations are exposed, so that it is difficult to restore the function of the device to its original form, and so that problems arise that may lead to the occurrence of emergency accidents or the like. Currently, therefore, from such a standpoint, various types of tamperproof screws have been proposed that cannot easily be tightened or removed by means of plus screwdrivers, minus screwdrivers or the like (screw turning tools) that are commonly marketed and used.
For example, a “screw” comprising a construction in which a three-forked groove is formed in the head of the screw, both sides of this groove are formed as inclined surfaces so that the groove becomes narrower as the groove becomes deeper, and the end surfaces of the groove are inclined by a specified angle (an angle smaller than 35°) with respect to the center line so that the groove becomes deeper approaching the center of the groove, has been proposed (see Patent Reference 1). More specifically, in the screw described in this Patent Reference 1, the following merits are obtained as a result of the use of the above-described structure: namely, the screwdriver bit that engages with the screw also has a thicker root, and the tip end of this bit has a thin shape; accordingly, there is no damage of the vanes of the screwdriver bit, and an accurate tightening torque can be obtained. Furthermore, since the groove of the screw has a broad width, engagement between the groove and the vanes of the screwdriver bit is relatively easy. Moreover, since the shape of the punch that is used for the press molding of the groove of the screw is also similar to the shape of the tip end of the screwdriver bit, the useful life of the punch is also extended.
Furthermore, a “tamperproof small screw” (see Patent Reference 2) has been proposed in which a Y-shaped groove is formed in the radial direction at a uniform spacing in the circumferential direction in the head of the screw, a circular conical hole that is centered on the intersection point of the respective branching grooves is formed at this intersection point, both side walls of the groove are raised directly upward from substantially the bottom surface of the groove, the end parts of the side walls of mutually adjacent branching grooves on the side of the intersection point are connected by rectilinear guide walls, and the bottom surfaces of the branching grooves are formed as inclined surfaces so that these branching grooves become shallower moving away from the center. More specifically, the screw described in this Patent Reference 2 has the following effect: namely, there is no protruding part that protrudes into the circular conical hole; and even if an attempt is made to loosen the screw using a minus screwdriver bit (rather than using a special screwdriver bit), the tip end of the screwdriver bit does not engage with the groove, so that the screw cannot be loosened. Furthermore, the G dimension (diameter of the track circle formed by the intersecting line between the bottom surface of the three-directional groove and the circular conical hole) and the B dimension (diameter of the inscribed circle of the circular arc surface that connects the mutually adjacent branching grooves inside the track circle) are substantially the same dimension. Accordingly, the following merits are obtained: namely, since the attachment roots of the vanes of the screwdriver bit are not slender, the vanes of the screwdriver bit are not damaged during the tightening of the screw or the like.
Meanwhile, a screw tightening structure (see Patent Reference 3) has been proposed which is installed on a screw and a tightening tool that is used to tighten this screw, which has a three-forked shape having three torque transmitting parts that respectively protrude toward the outer circumference at 120-degree intervals about the center line, and which comprises an engaging hole and an engaging protruding part that engage with each other, this screw tightening structure being characterized in that (1) the torque transmitting parts have a pair of rectilinear side parts that are parallel to the direction of protrusion from the center line, (2) adjacent torque transmitting parts are respectively smoothly connected via circular arc parts that have a fixed curvature radius R, and (3) where g is the diameter of the circumscribed circle of the tip ends of the above-described three torque transmitting parts, and b is the diameter of the inscribed circle of the three above-described circular arc parts, the structure is set so that both (4) the equation 0.5 g≦b≦0.6 g, and (5) the equation 0.5b≦R≦0.6b, are satisfied. More specifically, in the screw tightening structure described in this Patent Reference 3, as a result of the use of the above-described construction, the following effect is obtained: namely, damage to the engaging protruding part and deformation of the engaging hole can both be suppressed, and the screw can be tightened with a higher tightening torque.
Furthermore, in order to obtain a “screw” and screwdriver bit in which the phenomenon of “come-out” tends not to occur, a screw, which has a rotating part comprising a vane engaging part that engages the vanes of the screwdriver bit and which is characterized in that the part on the tip end side of the side wall surface on one or both sides of each vane engaging part is bent in the direction moving away from the side wall surface on the opposite side with respect to the part on the base end side of this side wall surface, and a screwdriver bit, which is comprised of a vane part which engages with the vane engaging part of the above-described rotating part of this screw, have been proposed (see Patent Reference 4). More specifically, in the screw and screwdriver bit described in this Patent Reference 4, as a result of the use of the above-described construction, the effect as described below is obtained: namely, the screwdriver bit does not readily tilt with respect to the screw; furthermore, the side wall surfaces of the vane engaging part that are bent in a shape that corresponds to the bent side surfaces of the vane part bite into these side surfaces. Accordingly, the above-described side surfaces can securely grip the above-described side wall surfaces, and as a result, the engagement between the vane part and the vane engaging part is good, thus making it difficult for “come-out” of the screwdriver bit to occur.
Furthermore, a screw with a thin head which is devised so that this screw is not subjected to the propulsion force of the screwdriver bit during tightening when the screw is tightened in a relatively thin, plate-form workpiece, and which is devised so that tightening and loosening of the screw can only be accomplished using a special screwdriver bit has been proposed; and this screw with a thin head is characterized in that (1) the head which has an engaging groove is formed with a relatively small thickness, (2) a reinforcing part having a tapered shape that is large on the head side and small on the screw threads side is formed between the seating surface of the head and the screw threads of this head, and (3) the wall surface engaged by the screwdriver bit during rotation in at least the tightening direction of the above-described engaging groove is formed as a planar surface that is substantially parallel to the axial line of the screw (see Patent Reference 5). More specifically, in the screw with a thin head described in this Patent Reference 5, as a result of the use of the above-described construction, such effects are obtained that the reinforcing part prevents a reduction in the thickness of the neck part with the engaging groove even in working that reduces the thickness of the head by press working, without any loss of the inherent function of the screw as a compact part but having a certain thickness that is used on relatively thin products; accordingly, the phenomenon of “neck stripping” of the screw does not occur, so that the function of this screw as a screw can be sufficiently manifested; and even when the screw is used in the assembly of precision electronic devices, the screw cannot be loosened unless a special screwdriver bit is used, so that the danger of unforeseen accidents occurring due to erroneous repairs performed by the user is small, and the screw thus also shows promise as a preventive measure.
Patent Reference 1: Japanese Utility Model Application Laid-Open (Kokai) No. H4-62408
Patent Reference 2: Japanese Patent Application Laid-Open (Kokai) No. H10-30625
Patent Reference 3: Japanese Patent No. 3026965
Patent Reference 4: Japanese Patent Application Laid-Open (Kokai) No. 2000-230526
Patent Reference 5: Japanese Patent Application Laid-Open (Kokai) No. 2001-280324